The invention concerns improvements in an assembly for the electro-deposit of aluminum onto base surfaces or substrate goods including a plating tank covered by a hood containing inert gas, such that smoke formation and the like arising from the plating cells of the tank are avoided.
A known device for electro-deposition of aluminum from aprotic, oxygen-free and water-free, aluminum-organic electrolyte, comprises an electroplating tank closed to ambient by means of a cover hood defining a space charged with inert gas. Substrate goods carriers are conveyed into the electroplating tank with the assistance of transport means over a charging liquid lock connected to the closing hood, passed by truck means into and out of the electroplating tank, and discharged with the assistance of further transport means over a discharge liquid lock likewise connected to the closing hood. The electroplating tank is subdivided into a plurality of identical individual cells rectangular in cross-section, which are disposed behind one another, such that the goods carriers are insertable and removable by a grip means allocated to all individual cells. The charging and discharging locks are disposed in series with the individual cells. A device of this type is described in detail in the German OS No. 31 33 162.
In this and similar systems, an electrolyte which consists essentially of aluminum triethyl, an aromatic hydrocarbon, and of an alkali fluoride is preferably employed. A majority of the aluminum triethyl becomes a free agent solid in the form of a complex. Such an electrolyte, however, reacts with water as well as with air, upon formation of solids. An electrolyte temperature of approximately 100.degree. C. is employed in order to obtain as high as possible a deposition rate. This results in the fact that, due to the partial vapor pressures of the individual components of the electrolyte system, considerable amounts of solvent, and aluminum alkyl under certain conditions, pass into the closing hood space filled with inert gas. Larger amounts of solvent quickly evaporate from the electrolyte dampened goods carriers particularly when the goods carriers are removed from the electrolyte bath of the aluminum-plating cell so that even solid aluminum complex per se is entrained in a finely distributed form. A fine smoke thus arises as a result in the essentially oxygen-free and water-free inert gas space within the closing hood. This smoke also develops as a result of the reaction of the aluminum alkyl with the slight amounts of air and humidity present in the gas space. However, this smoke formation must be avoided in a plating system since it can have a negative effect on the adhesion and porosity of the aluminum coating.
A further problem also occurs in known devices of the above type. A fluid, usually a solvent, which usually has a lower vapor pressure at room temperature than the toluol in the approximately 100.degree. C. hot aluminum-plating bath is situated as the sealing liquid in the fluid lock of the charging and discharging locks. Particularly in a large bath, solvent vapors and aluminum alkyl vapors cannot be prevented from proceeding into the fluid locks. This movement of vapors, however, must be prevented because of the danger of evaporation of the solvent out of the electrolyte and since irreversible chemical reactions in the liquid locks can lead to sludge formation in the lock due to precipitation of insoluble aluminum compounds. Thus, this vapor movement unchecked can lead to poor deposition quality.
One attempt to resolve the problems described above has been made by disposing cooling coils inside of the closing hood. The condensed aluminum electrolyte vapors are thereby subsequently returned into the electroplating bath. The transport of the electrolyte toluol and of the aluminum alkyl into the fluid locks can indeed be prevented with this measure but the smoke formation is not eliminated.
Further, due to non-governable temperature gradients between the sealing liquid and the electroplating bath on the one hand and the cooled space inside the closing hood on the other hand, a more or less considerable mass tramster of vapors naturally eventually occurs over a time span.
It is an object of the invention to improve the plating assembly of the type initially cited such that contamination of the liquid locks and of the gas space above the electrolyte with aluminum alkyl is reliably prevented and uncontrollable evaporation of larger amounts of solvent from the aluminum electrolyte is avoided.